TRUNK UEQUIREMENTS IX ALTKKXATE ROI^TIXG NETWORKS 293 



load offered to the P01-P02 group is composed in part of the overflow 

 traffic from TC1-P()2. It is likewise clear that the linal group TCl-POl 

 will l)c offered overflow traffic from both T(M-T(^2, TCf-P()2 and TCl- 

 SC and hence the final group can not he. enginetn'ed until the overflows 

 from all high usage grt)ups terminating at TCI have been detcrminech 

 This transfer of load from group to group with each succeeding high 

 usage group ser\ ing as the base of a lunv triangle in orderly procession 

 from TC to IvC is the essence of the multi-alternate routing system and, 

 therefore, a i)recise order of group load computation was necessary to 

 assun> propel' accounting of the load offered to each group. In practice 

 the order of group load computation retjuires that all TC-TC high usage 

 groups be established first, then TC-PO groups, TC-SC and so on. The 

 process in^'olves selecting a particular TC as a "reference" and examin- 

 ing all traffic invoh'ing that TC to determine what (if any) high usage 

 groups should terminate there. 



The second rule, affecting the concentration of switched traffic, is 

 related to the order of group load computation in that it postulates a 

 starting point for the process of examining HU group possibilities. The 

 need for such a rule may be best explained by reference to Fig. 7 repre- 

 senting a simple intertoll network in which PO, a CSP serving four 

 TC's, homes on SC, another CSP, serving four other TC's. The question, 

 answered by the second rule, is which of the eight TC's should be used 

 as the first reference TC. The choice will determine the sizes of, i.e., 

 the number of trunk terminations to be provided at, PO and SC, respec- 

 tively. Let us examine the reason for this. Assume the TC's homing on 

 PO are to be used as the first set of reference TC's (it makes no difference 

 which TC is first cho-sen). Assume that the investigation showed an eco- 

 nomical HU group between TC2 and SC Ixit that no HU groups proved 

 into any of the TC's (5, 6, 7, and 8) dependent upon SC. The load offered 



